The overall goal of this proposal is to understand the structural basis of specificity and mechanism in several C-type animal lectins that function in innate and adaptive immunity. 1) Mannose-binding proteins (MBPs) recognize oligosaccharide structures present on pathogenic cell surfaces and trigger killing of these organisms. When an MBP oligomer binds to a target surface, MBP-Associated Serine Proteases, or MASPs, are activated, which in turn activate downstream complement components that neutralize the pathogen. The mechanism of MASP activation by MBP will be probed by determining the structure of a MASP fragment that contains full MBP-binding activity, alone and in complex with MBPs, to visualize directly their interactions and consequent conformational changes in the two proteins. Full-length MASP bound to MBP will ultimately be used to understand the activation of the serine protease domain. 2) DC-SIGN is a receptor present on antigen-presenting dendritic cells which mediates their initial interactions with T cells. DC-SIGN and a related receptor, DC-SIGNR, greatly enhance the infection of T cells by human immunodeficiency virus (HIV). In order to understand the specificity of DC-SIGN and DCSIGNR towards high-mannose oligosaccharides and glycoproteins, structures of complexes between these proteins and high-mannose oligosaccharides will be determined. Tetrameric constructs representing the complete extracellular domain of these proteins will be studied in order to assess the contribution of the tetramer to high-avidity binding and/or counter-receptor crosslinking. To probe the binding site further for inhibitor design, phage display methodology will be used to discover peptides that can bind strongly and selectively to DC-SIGN. The structures of strongly binding peptides bound to DC-SIGN will be determined.